Multiple access communication system schemes such as frequency division multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA) are designed to eliminate or reduce signal interference between multiple users. In real-world applications, however, signals received over a wireless medium in a multiple access communication system suffer from signal degradation caused by a number of factors including multiple access interference (MAI), intersymbol interference (ISI), and adjacent cell interference (ACI). MAI and ISI are by-products of multipath fading, a phenomenon in which an originally transmitted signal and multiple reflections of the originally transmitted signal arrive at a receiver at different times. Multipath fading degrades system performance because each reflected signal experiences attenuation, a time delay, and a possible frequency shift before being received at the receiver. ACI is a by-product of a multi-cell environment wherein a signal in a subject cell is distorted by noise introduced from a signal in a neighboring cell. Each cell in a multi-cell environment defines an area in which an associated base station manages signal communications. Typically, the managing area extends in a radial direction from the location of the base station. The area covered by a cell is dependent upon a number of factors including, for example, signal degradation due to adjacent cell interference and multipath interference caused by environmental structures.
Numerous designs have been proposed to reduce multi-user interference and the adverse manner in which it effects signal reception. High expense and component complexity associated with implementing such designs, however, limit their effectiveness. Conventional CDMA systems using a rake receiver, for example, require channel estimation processing, code acquisition and tracking, fast power control, and frequent hand-over for every path of a received signal. The above-mentioned systems generally require numerous redundant overhead parameters such as synchronization and pilot channels to reduce the complexity of signal processing. When these systems are implemented, however, the foregoing parameters have little to no effect in reducing or eliminating interference.
Another conventional CDMA system uses large area synchronous code-division multiple access (LAS-CDMA) technology for reducing interference. LAS-CDMA implements smart coding and an interference reduction window to reduce the impact that multiple access interference, intersignal interference, and adjacent cell interference have on signal communication. In generating smart codes, an LAS-CDMA receiver establishes an interference reduction window by transmitting codes with an offset auto-correlation and cross correlation coefficient equal to zero. The interference reduction window can provide a solution for reducing multi-path interference, however, LAS-CDMA fails to reduce or eliminate intersymbol interference. Moreover, LAS-CDMA technology reduces intercell interference but at the expense of a significant reduction in the space allotted for user codes. A further shortcoming of LAS-CDMA technology involves a reduction in channel and system capacity.
In light of the aforementioned problems encountered by existing multi-cell multiple access communication systems there is a need for a multi-cell CDMA system that can significantly reduce the effects of interference and at the same time simplify the CDMA system design.